Weathering, Soil, and Erosion Chapter 5

Weathering  Def: the breakup of rock due to exposure to processes that occur at Earth’s surface  2 types:  Mechanical- the breakdown of rock that takes place when a rock is broken into smaller pieces of the same material without changing its composition  Chemical- the breakdown or decomposition of rock that takes place when minerals are changed into different substances

Mechanical Weathering  Processes  Frost wedging  Def: a process in which water freezes in the cracks of a rock and wedges it apart  Mostly in porous rocks and in rocks with many cracks  In places where there are frequent freezes and thaws  Ice Heaving- creates potholes; ice lifts pavement up and it collapses when it thaws

Examples of Frost Wedging and Ice Heaving

Mechanical Weathering cont…  Abrasion  Def: the wearing away of rock material by grinding action  Water, wind and ice are capable of moving rocks.  Sand is a product of abrasion.

Mechanical Weathering cont…  Plants and Animals  Growth contributes  Roots wedged into tiny pores and crevices  Burrowing contributes  Digging holes into soil  Bringing rock fragments to surface

Mechanical Weathering cont…  Upward Expansion  Exfoliation- the peeling of surface layers from exposed bedrock  Rock is uplifted by tectonic forces (ex: granite)  Overlying rock is worn away which reduces pressure  Upward expansion causes the granite to break along curved joints  Large sheets of loosened rock break away

Upward Expansion Examples

Chemical Weathering  Water  Hydrolysis- the chemical weathering by reaction of water with other substances  Acids: seeps into the ground and reacts chemically with many common minerals  Calcite  Dissolves completely  Underground caverns

Chemical Weathering cont…  Acid Rain  Def: rainwater that contains unusually high amounts of acids that can be traced back to pollutants  Increases the rate of chemical weathering

Chemical Weathering cont…  Oxidation  Def: chemical reaction of oxygen with other substances  Effective with minerals containing iron  Formation of rust, or iron oxides

Rates of Weathering  Weathering is usually a slow process  Factors that affect the rate of weathering:  Surface Exposure  More surface exposed, weathers more quickly  Composition of Rock  Various rocks are affected differently by weathering processes  Climate  Warm/wet climates: both chemical and mechanical  Cold/dry climates: mainly mechanical

Soil  Def: loose, weathered rock and organic material in which plants with roots can grow  Formed by weathering  Contains air, water, organic material, mineral matter  Parent material  Def: material from which a soil is formed  Residual soil- soil whose parent material is the bedrock below  Transported soil- soils formed from transported materials  Deposits left by winds, rivers, and glaciers

Soil Profile  Def: cross section of earth exposed by the digging  Soil horizons- three distinct zones of mature soil  A-horizon- topsoil  Darker, contains humus (organic material), sandy  B-horizon- subsoil  Red or brown, iron oxides, clay from topsoil, calcium and magnesium  C-horizon  Weathered parent material, rock fragments

Soil Composition  3 noticeable parts: sand, silt, and clay  Affects soil’s ability to hold water and air  Factors that affect composition:  Time  Parent material  Plants and animals  Topography  Climate- MOST IMPORTANT!

12.3 Mass Movements and Erosion  Mass movements- the downward transportation of weathered materials  Gravity causes materials to fall, slide, or move at slow speeds to lower levels  Erosion- the removal and transport of materials by natural agents such as wind and running water

Mass Movements  Talus- rock fragments that have been weathered from a cliff and pulled down by gravity  Landslide- movement of a mass of bedrock or loose soil and rock down the slope of a hill, mountain, or cliff  Steep slopes  Regions near volcanoes and in earthquake-prone regions

Mass Movements  Creep- slow, imperceptible movement of soil down a slope  Causes fence posts, poles and other objects fixed in soil to lean downhill  Slump- a block of land tilts and moves downhill along a surface that curves into the slope  Tends to occur because bottom of slope can no longer support top of slope

Mass Movements  Earthflows- the downslope movement of a mass of earth materials that have been saturated with water  Slower and less fluid than a mudflow; velocity affected by amount of water present, the composition of the soil, and the steepness of the slope  Mudflows- the downslope movement of water that contains large amounts of suspended clay and silt  Rapid movement; capable of moving rocks, boulders, trees, and houses; lahars- volcanic mudflows

Erosion  Topography- uplifting and weathering/erosion are happening at same time  More uplifting- rugged and sharp (Himalayas)  More erosion- smooth and rounded (Appalachians)  Climate- in humid areas, water is primary agent and causes rounded topography  Composition of rock- some types are more resistant than other rock types  Volcanic neck- the solidified lava filling the center vent of an extinct volcano (picture pg 270)